Research Concepts RC2500 - Manual (Page 36)

RC2500 Antenna Controller Chapter 4 Inclined Orbit Satellites 29
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• SEARCH..receives control if the satellite signal is lost.
• ERROR..receives control if any error is encountered.
4.2.2 PROGRAM_TRACK
When the PROGRAM_TRACK submode is active, the controller smoothly positions the antenna to
azimuth and elevation positions derived from the track table, regardless of whether a satellite signal is
present or not. The track table holds a map of the satellite's apparent motion as a function of sidereal
time. The controller performs a linear interpolation between adjacent track table entries to determine the
correct antenna position.
In PROGRAM_TRACK submode antenna movement occurs whenever the controller calculates that the
error between the antenna's current position and the antenna's proper position as determined from the
track table exceeds Max Track Error tenths of a dB. To determine the error between the current and
calculated antenna position, the controller calculates the antenna beamwidth based on the Antenna Size
and the frequency Band. If the Max Track Error parameter is decreased in value, the antenna pointing
error will be less, but the antenna will move more often, which increases wear on the antenna actuators.
The PROGRAM_TRACK submode also periodically performs peakups (similar to STEP_TRACK
submode) to check the accuracy of track table entries. If the discrepancy between the peakup position
and the position stored in the track table is greater than that specified by the Max Track Error parameter,
the update flag for each entry in the track table is set TRUE. The frequency at which these track table
accuracy checks occur is determined by the Update Check parameter. The default Update Check
interval is 33 hours, but can be changed by the user to any value in the range of 1 to 999 hours (via the
TRACK MENU system).
If the current sidereal time reaches a value that corresponds to an entry in the track table whose update
flag is set TRUE, and the current signal strength level indicates that a satellite signal is present, a peakup
will occur, the track table entry will be updated, and the track table update flag is reset FALSE. If the
satellite signal is not present the update flag remains set TRUE and the system will try again in 23 hours,
56 minutes, and 4 seconds. The user can view the contents of each track table entry and examine the
present state of the update flag. In addition the user can invoke the Force Update or Reset Update
functions to force the update flag for each entry in the track table to be either set TRUE or reset FALSE,
respectively. These functions are accessible via the TRACK MENU system, which is described in more
detail in section 5.4.
The PROGRAM_TRACK submode will pass control to the other TRACK mode submodes under the
following circumstances:
STEP_TRACK..receives control when the current sidereal time reaches a value for which there
are no valid track table entries and a valid satellite signal is present.
SEARCH..receives control when the current sidereal time reaches a value for which there are no
valid track table entries and a valid satellite signal is not present.
ERROR..receives control when any error is encountered.
4.2.3 Intelli-Search
The SEARCH submode receives control whenever the satellite signal is lost and the track table does not
contain antenna azimuth and elevation position data for the current sidereal time. Whenever the
SEARCH submode is active the controller periodically performs a search over the region where the
controller has calculated that the satellite will be found. The Intelli-Search algorithm determines the
extent of the search region based on the Satellite Longitude, the Antenna Latitude and Antenna
Longitude, the antenna beamwidth (as calculated from the Antenna Size and frequency Band), and the
Elevation and Azimuth Scale Factors.
The Azimuth and Elevation scale factors are fixed and refer to the mapping of antenna azimuth and
elevation position counts to antenna azimuth and elevation angles, respectively. The 360° (or 2*PI)
angular region measured by the resolvers and on-board resolver-to-digital converters is divided into 2 16
(or 65,536) discrete positions. The azimuth and elevation scale factors are the number of counts in one
radian of angle for the two axes. This value is 10,430 counts/radian for both Az and El. One ‘count’ is
equivalent to 360 / 65,536 = 0.005493°. This number represents the resolution of the angular